The invention relates to a method of making a screen cylinder, in which method screen wires are arranged next to each other at predetermined intervals and attached parallel to the axis of the screen cylinder onto the inner surface of support hoops, and the support hoops are then bent to form an annular shape, whereby the screen wires provide a cylindrical screen surface on the inside of the support hoops.
The invention also relates to a screen cylinder for cleaning or sorting a pulp mixture, the screen cylinder comprising screen wires arranged parallel to the axis of the screen cylinder at-predetermined intervals to provide a cylindrical screen surface, said screen wires being attached onto the inner surface of support hoops arranged around them.
Screen cylinders are used, for example, for cleaning and sorting a pulp mixture. They are manufactured by attaching parallel screen wires that provide a screen surface side by side in a cylindrical shape so that there remains a slot of a desired size between them. Generally this is performed by providing grooves in separate support wires at even intervals, and then arranging the screen wires in the grooves at even intervals. Subsequently, the screen wires are usually attached to the support wires by welding or brazing, and the so formed sheet-like unit is then bent to form a cylinder, and the ends of the support wires are joined by welding so that they form support hoops. The screen cylinders, however, are difficult and slow to manufacture in the known manner. In addition, the welding produces burrs, spatters and the like where the fibres adhere to and then detach from as larger masses, thereby causing problems in the steps that follow.
Further, EP 0 705 649 discloses a method of making a screen cylinder based on an interlocking fit. In the solution of the publication, the profile of the screen wires, and the mounting grooves provided in the support hoops are such that the rods inserted in the grooves stay in the grooves when the support hoops are bent. The profile of the screen wires, and the grooves of the support hoop, however, are expensive to make. Any deflections caused by inaccuracy in the manufacture easily impair the attachment, which in turn produces vibration that is harmful to the operation and durability of the screen cylinder.
The present invention seeks to overcome the disadvantages noted above, by providing a screen cylinder and method of making a screen cylinder in which the screen wires are attached to each support wire by welding them thereto from the side of the support wire facing away from the screen wire in such a way that the weld extends to the screen wire, thereby firmly attaching each screen wire to the support wire. Accordingly, the visible portion of the weld lies on the outside of the screen cylinder, and hence any burrs or the like resulting from the welding operation will not impair the performance of the screen cylinder.
Mounting grooves are preferably provided for the screen wires at suitable intervals on the inner surface of the support wire, i.e., on the surface that is on the inside of a finished screen cylinder. The screen wires are inserted into the grooves, after which they are attached to the support wire by a weld provided from the outside of the support wire, i.e., from the side facing away from the mounting grooves. After the attachment, a cylindrical structure is formed by bending the support wires to form support hoops. In one embodiment of the invention, the weld joining a screen wire to the support wire is provided from the outside of the support wire by using a welding method with sufficiently deep penetration so that the weld extends through the thickness of the support wire into the screen wire. In another embodiment, a welding groove extending partway through the thickness of the support wire toward the mounting grooves is provided on the outer surface of the support wire, and the welding is performed at the welding groove. This enables a continuous weld to be effected along the welding groove to secure all of the screen wires to the support wire. In yet another embodiment, the mounting grooves on the inside of the support wire and the welding groove on the outside intersect so that a screen wire arranged in the mounting groove is visible from the side of the welding groove and can be attached at the groove by a weld. The welding is thus not performed through the support wire.
The advantage of the invention is that the welding is performed on that side of the support wire which will be on the outer surface of the screen cylinder. Consequently, no rough points, bulges or the like are formed on the inside of the screen cylinder as a result of the attachment of the wires. Furthermore, the weld does not necessarily need to be ground or otherwise finished to remove rough points, which means that this slow and expensive finishing step can be eliminated. Where the support wire includes the welding groove, the support wire can be fairly broad and therefore rigid. Due to the welding groove, the weld can be provided such that the penetration of the welding is sufficient to firmly attach a screen wire to the support wire. Further, as noted above, a continuous weld can be made along the bottom of the welding groove, whereby the weld need not be aligned separately at each screen wire. The weld is thus quick to make, and the action can be easily automated.